Project Summary. Five-membered carbocycles are featured prominently in a large number of biologically important molecules such as prostaglandins, steroids, nucleic acids, peptide nucleic acids and ACE inhibitors. However, there are few methods amenable to their efficient synthesis. The Pauson-Khand reaction (PKR) is among the most attractive approaches to cyclopentenones, as the three components in the [2+2+1] cycloaddition are often easily accessible or commercially available. Despite four decades of research, the intermolecular variant of the PKR has serious limitations. Herein, we propose alternate acetylene sources to circumvent many of the reported limitations. We will also study the mechanism by which the PKR occurs. One of the limitations of the intermolecular, three-component PKR is related to the regioselectivity of alkyne incorporation into the product cyclopentanone. Addressing this lack of regioselectivity is also the most understudied aspect of improvements to intermolecular PKRs. It is known that terminal acetylenes react with complete regioselectivity to yield the ?-substituted cyclopentenone; however, accessing the ?-substituted cyclopentenone directly is impossible. Internal, unsymmetrical alkynes typically react with poor regioselectivity yielding mixtures of products. Moreover, terminal and unsymmetrical alkynes can be expensive or only accessible via multistep syntheses. We propose the use of simple and inexpensive alkynes to circumvent both of these problems. The application of calcium carbide and trichloroethylene in PKRs will give access to easily functionalizable cyclopentenones, important in natural product and drug synthesis. The initial mechanism proposed 30 years ago has only indirect support. Gaining a detailed understanding of the mechanism will point to ways to improve the PKR. We will therefore synthesize a number of organocobalt complexes, trap out intermediates along the reaction pathway, and subsequently study their electronic and geometric structures via X-ray absorption spectroscopy (XAS). These results will then be directly correlated with their reactivity to gain a deeper picture of the PKR mechanism and its implications. Our research has broad applications in synthetic organic and organometallic chemistry. Furthermore, these spectroscopic studies on reactive intermediates will shed light on the behavior and reactivity of organocobalt complexes in general. We expect that the studies from this grant will significantly expand the scope, understanding and utility of the PKR. As the use of XAS as a tool to study homogenous catalysis is underutilized, this work will also highlight the application of XAS to the study of reactive organometallic reactions in general.